Many oil wells employ a downhole reciprocating type production pump in order to lift fluid from the lower extremity of a borehole to the surface of the ground. Joints of rod are made up into a string which extends from the surface of the ground downhole to a production pump so as to enable a pump jack located on the surface of the ground to reciprocate the rod which in turn actuates the downhole production pump. From time to time, for one reason or another, the rod must be removed from the borehole. It is customary upon removal of the rod from the borehole to subject the individual joints of rod to various analyses and tests, and to recondition the rod so as to enable the structurally sound rod to be returned to service.
Replacement of sucker rod in a well bore is an expensive endeavor for the reason that the rod is often made up into a continuous string which exceeds ten thousand feet in length, and therefore special equipment and trained personnel are required when the replacement becomes necessary.
Some joints of sucker rod enjoy a life of many years while other joints experience a limited life of only a few days duration. Many people skilled in the art of sucker rod theorize that rod which is properly reclaimed is more reliable than new joints of rod.
It is old in the art to degrease sucker rod, to clean and surface harden rod by shot peening, or to subject the rod to electro-magnetic inspection, called "magnaflux". After the rod has been cleaned, surface hardened and inspected, it is usually painted and stored for subsequent use. It has been found that many joints of sucker rod treated in this manner enjoy several additional years life when again placed into service in a borehole. Yet, for some reason heretofore unknown, one or more joints of new or reclaimed rod will sometimes fail, although it appears to be structurally sound. When an entire string of sucker rod has been removed from a well bore and subjected to analyses, it often is discovered that the structural integrity of a few joints has degenerated because small cracks or fractures have developed, or that the corrosive environment of the well has deteriorated the rod so as to render it useless.
Hydrogen ion embrittlement is a phenomenon which occurs when case-hardened metallic goods are placed in an acid environment. It is believed that the hydrogen ion found in acid solutions is sufficiently small in size to physically enter small crevasses or fissures at or near the surface of the metallic goods, thereby progressively opening the crack or fissure, and further encouraging the hydrogen ion intrusion. The phenomenon continues in this manner, much like a wedge being driven into an existing void or crack.
In manufacturing sucker rod, the metal composition will sometime appreciably change along the length of the rod for reasons best understood and appreciated by the metallurgist. It is desirable to ascertain the magnitude of this metallic composition change and to eliminate those rods wherein the change in composition could possibly adversely affect the structural integrity of the rod.